Stray-field robust, twist-insensitive magnetic speed sensors

ABSTRACT

A magnetic sensor module includes a magnetic sensor having an in-plane axis and an out-of-plane axis, and including a differential pair of sensor elements spaced apart from each other. The differential pair of sensor elements are configured to generate measurement values in response to sensing a bias magnetic field. The magnetic sensor module further includes a back bias magnet including two opposing poles, where the back bias magnet is magnetized in a magnetized direction that is parallel to the in-plane axis and generates the bias magnetic field; a first magnetic flux guide disposed at a first pole and configured to redirect a first portion of the bias magnetic field towards the magnetic sensor along the in-plane axis; and a second magnetic flux guide disposed at a second pole and configured to redirect a second portion of the bias magnetic field towards the back bias magnet along the magnetized direction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/403,790 filed on Jan. 11, 2017, the contents of which are herebyincorporated by reference herein.

FIELD

The present disclosure relates generally to sensing a wheel speed, and,more particularly, to magnetic speed sensors.

BACKGROUND

To measure wheel speed (e.g., in an automotive application) typically aferromagnetic wheel is used in combination with a magnetic sensitivesensor and a magnet mounted to the sensor. The sensor generatesoutput-pulses. A control unit counts the pulses and is able to calculatewheel-speed and actual angle of the rotating wheel.

In camshaft sensing applications, a Hall monocell configuration may beused that enables output switching at the tooth edge of a toothed wheel.A z-magnetized back bias sensor in combination with the Bz-sensitivemonocell sensor generates a sinusoidal signal as the ferrous targetwheel rotates in front of the sensor. The maximum amplitude is achievedwhen a tooth passes the sensor, while the minimum signal is achievedwhen the sensor faces a notch of the toothed wheel. Thus, the sensordevice switches on the tooth edge.

A benefit in using a Hall monocell sensor is that the sensor istwist-insensitive such that the the sensor will work independent from amounting position regardless of its rotational orientation around itsz-axis. Thus, an air-gap between the sensor module and the wheel can beadjusted during mounting using a screw. That is, twisting the sensormodule using the screw will adjust the air gap and the rotationalorientation of the sensor can be disregarded. Accordingly, the assemblytolerances are relaxed during mounting of the sensor due to thetwist-insensitivity.

On the downside, Hall monocell sensors have a disadvantage in terms ofstray-field robustness. Stray-fields are magnetic fields that areintroduced by external means located in the proximal environment of thesensor. For example, components located within a vehicle (e.g., forhybrid cars due to current rails driving high electrical currents closeto the sensing device or due to inductive battery charging) or acurrents flowing through a railway of a train system that generatesmagnetic fields may cause stray-field disturbance.

Alternative to the Hall monocell sensor, differential Hall sensingelements may be used to increase the stray-field robustness. In adifferential Hall sensor, two Hall plates are spaced apart. The outputsignal is calculated by subtracting the Bz signal of the first Hallplate from the Bz signal of the second Hall plate, and a homogeneousstray-field in the z-direction will cancel out due to the differentialcalculation.

The differential Hall signal has its signal maximum at the rising edgeof a tooth of the wheel and its signal minimum at the falling edge of atooth of the wheel. Thus, in contrast to the Hall monocell sensor, theoutput of the differential Hall sensor switches on the tooth center andthe notch center.

However, because the switching point is different, a vehicle'selectronic control unit (ECU) needs to be reconfigured to adjust theswitching point. Furthermore, another disadvantage of the differentialHall sensor is that it is not twist-insensitive. Twisting the sensormodule around its z-axis, will result in a decreasing signal. The worstcase is a twist angle of 90°, where both Hall plates sense the sameBz-field. In this case no differential signal is available and thesensor is not able to detect a tooth or a notch.

Therefore, an improved device that is both twist-intolerant may bedesirable.

SUMMARY

Magnetic sensor modules, systems and methods are provided, configured todetect a rotation of an object, and, and more particularly, to detect aspeed of rotation of an object.

Embodiments provide a magnetic sensor module including a magnetic sensorhaving an in-plane axis and an out-of-plane axis, and includes adifferential pair of sensor elements spaced apart from each other. Thedifferential pair of sensor elements are configured to generatemeasurement values in response to sensing a bias magnetic field. Themagnetic sensor module further includes a back bias magnet including twoopposing poles, where the back bias magnet is magnetized in a magnetizeddirection that is parallel to the in-plane axis and generates the biasmagnetic field; a first magnetic flux guide disposed at a first pole andconfigured to redirect a first portion of the bias magnetic fieldtowards the magnetic sensor along the in-plane axis; and a secondmagnetic flux guide disposed at a second pole and configured to redirecta second portion of the bias magnetic field towards the back bias magnetalong the magnetized direction.

Embodiments further provide a method of measuring a rotational speed ofa rotating member by a magnetic sensor, the method including generatinga bias magnetic field that is directed in parallel to an in-plane axisof the magnetic sensor; redirecting a first portion of the bias magneticfield towards the magnetic sensor along the in-plane axis; redirecting asecond portion of the bias magnetic field such that flux lines of thebias magnetic field from the redirected first portion of the biasmagnetic field and flux lines of the bias magnetic field from theredirected second portion of the bias magnetic field are coupled in aloop; generating measurement values by a differential pair of sensorelements in response to sensing an out-of-plane component of the biasmagnetic field, where variations in the measurement values generated bythe differential pair of sensor elements are caused by a rotation of theobject; generating a differential measurement signal based on themeasurement values generated by the differential pair of sensorelements, where the differential measurement signal oscillates betweentwo extrema as the object rotates; and outputting the differentialmeasurement signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are described herein making reference to the appendeddrawings.

FIGS. 1A and 1B illustrate a magnetic field sensing principle of atoothed wheel according to one or more embodiments;

FIG. 2 illustrates a plan view of a sensor arrangement according to oneor more embodiments;

FIG. 3 illustrates a back bias magnet that is an axially polarizedcylinder according to one or more embodiments;

FIG. 4 illustrates a speed sensing system according to one or moreembodiments;

FIGS. 5A-5D illustrate output signals of a sensor circuit of a sensorshown in FIG. 4 verse a rotation angle of a target wheel according toone or more embodiments;

FIG. 6 illustrates a sensor system according to one or more embodiments;

FIG. 7 illustrates a sensor module according to one or more embodiments;

FIG. 8 illustrates a plan view of the sensor module of FIG. 7 andshowing magnetic field patterns coupled in a loop in an environmentaround the sensor module according to one or more embodiments;

FIGS. 9A-9D illustrate an output signals of a sensor circuit of a sensorshown in FIG. 7 verse a rotation angle of a target wheel according toone or more embodiments; and

FIG. 10 illustrates a flow diagram of a method of measuring a rotationalspeed of a rotating member by a magnetic sensor according to one or moreembodiments.

DETAILED DESCRIPTION

In the following, a plurality of details are set forth to provide a morethorough explanation of the exemplary embodiments. However, it will beapparent to those skilled in the art that embodiments may be practicedwithout these specific details. In other instances, well-knownstructures and devices are shown in block diagram form or in a schematicview rather than in detail in order to avoid obscuring the embodiments.In addition, features of the different embodiments described hereinaftermay be combined with each other, unless specifically noted otherwise.

Further, equivalent or like elements or elements with equivalent or likefunctionality are denoted in the following description with equivalentor like reference numerals. As the same or functionally equivalentelements are given the same reference numbers in the figures, a repeateddescription for elements provided with the same reference numbers may beomitted. Hence, descriptions provided for elements having the same orlike reference numbers are mutually exchangeable.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.).

In embodiments described herein or shown in the drawings, any directelectrical connection or coupling, i.e., any connection or couplingwithout additional intervening elements, may also be implemented by anindirect connection or coupling, i.e., a connection or coupling with oneor more additional intervening elements, or vice versa, as long as thegeneral purpose of the connection or coupling, for example, to transmita certain kind of signal or to transmit a certain kind of information,is essentially maintained. Features from different embodiments may becombined to form further embodiments. For example, variations ormodifications described with respect to one of the embodiments may alsobe applicable to other embodiments unless noted to the contrary.

Signal conditioning, as used herein, refers to manipulating an analogsignal in such a way that the signal meets the requirements of a nextstage for further processing. Signal conditioning may include convertingfrom analog to digital (e.g., via an analog-to-digital converter),amplification, filtering, converting, biasing, range matching, isolationand any other processes required to make a sensor output suitable forprocessing after conditioning.

Embodiments relate to sensors and sensor systems, and to obtaininginformation about sensors and sensor systems. A sensor may refer to acomponent which converts a physical quantity to be measured to anelectric signal, for example, a current signal or a voltage signal. Thephysical quantity may for example comprise a magnetic field, an electricfield, a pressure, a force, a current or a voltage, but is not limitedthereto. A sensor device, as described herein, may be a current sensor,gauss meter, an angle sensor, a linear position sensor, a speed sensor,and the like.

A magnetic field sensor, for example, includes one or more magneticfield sensor elements that measure one or more characteristics of amagnetic field (e.g., an amount of magnetic field flux density, a fieldstrength, a field angle, a field direction, a field orientation, etc.)corresponding to detecting and/or measuring the magnetic field patternof an element that generates the magnetic field (e.g., a magnet, acurrent-carrying conductor (e.g. a wire), the Earth, or other magneticfield source).

According to one or more embodiments, a magnetic field sensor and asensor circuit are both accommodated (i.e., integrated) in the same chippackage (e.g., a plastic encapsulated package, such as leaded package orleadless package, or a surface mounted device (SMD)-package). This chippackage is also referred to as sensor package. The sensor package may becombined with a back bias magnet to form a sensor module, sensor device,or the like.

One or more magnetic field sensor elements, or for short a magneticfield sensors, included in the sensor package is thus exposed to themagnetic field, and the sensor signal (e.g., a voltage signal) providedby each magnetic field sensor element is proportional to the magnitudeof the magnetic field, for example. Further, it will be appreciated thatthe terms “sensor” and “sensing element” may be used interchangeablythroughout this description, and the terms “sensor signal” and“measurement value” may be used interchangeably throughout thisdescription.

The sensor circuit may be referred to as a signal processing circuitand/or a signal conditioning circuit that receives the signal (i.e.,sensor signal) from the magnetic field sensor element in the form of rawmeasurement data and derives, from the sensor signal, a measurementsignal that represents the magnetic field. The sensor circuit mayinclude a digital converter (ADC) that converts the analog signal fromthe one or more sensor elements to a digital signal. The sensor circuitmay also include a digital signal processor (DSP) that performs someprocessing on the digital signal, to be discussed below. Therefore, thesensor package comprises a circuit which conditions and amplifies thesmall signal of the magnetic field sensor via signal processing and/orconditioning.

A sensor device, as used herein, may refer to a device which includes asensor and sensor circuit as described above. A sensor device may beintegrated on a single semiconductor die (e.g., silicon die or chip),although, in other embodiments, a plurality of dies may be used forimplementing a sensor device. Thus, the sensor and the sensor circuitare disposed on either the same semiconductor die or on multiple dies inthe same package. For example, the sensor might be on one die and thesensor circuit on another die such that they are electrically connectedto each other within the package. In this case, the dies may becomprised of the same or different semiconductor materials, such as GaAsand Si, or the sensor might be sputtered to a ceramic or glass platelet,which is not a semiconductor.

Magnetic field sensor elements include, but is not limited to, Hallplates, vertical Hall effect devices, or magneto-resistive sensors,often referred to as XMR sensors which is a collective term foranisotropic magneto-resistive (AMR), giant magneto-resistive (GMR),tunneling magneto-resistive (TMR), etc.

FIGS. 1A and 1B illustrate a magnetic field sensing principle of atoothed wheel 1 that has alternating teeth 2 and notches 3 according toone or more embodiments. In particular, the toothed wheel 1 may be madeof a ferromagnetic material (e.g., iron) that attracts magnetic fields.In addition, a sensor arrangement 4 is configured to sense a magneticfield produced by an axially polarized back bias magnet 5, where thesensor arrangement 4 and the back bias magnet 5 comprise a sensor module6. The sensor arrangement 4 may generally be referred to herein as asensor and may be disposed in a sensor package. The axially polarizedmagnet 5 creates a radially symmetric bias magnetic field in the sensorpackage plane (i.e., chip plane). A diametrically polarized cylinder maybe used as the axially polarized magnet 5. The magnetic field producedby the axially polarized magnet 5 is zero at the center of the magnet(i.e., at its center axis) and increases in a radial direction from thecenter axis (e.g., the z-axis as shown). Thus, an in-plane magneticfield is created by the magnet 5.

Here, the sensor plane(s) of the sensor elements within the sensorarrangement 4 are arranged parallel to the in-plane components of themagnetic field. The sensor planes, as shown in FIGS. 1A and 1B, arealigned in the x and y-directions, perpendicular to each other, andrepresent the sensitivity-axis of the sensor elements such that thesensor elements are sensitive to the in-plane magnetic field componentBx (i.e., the magnetic field in the x-plane) or to the in-plane magneticfield component By (i.e., the magnetic field in the y-plane) of thesensor arrangement 4. Thus, the sensor elements are sensitive to theradially symmetric bias magnetic field produced by the magnet 5.

FIG. 1A shows a tooth 2 of wheel 1 passing the sensor module 6. In thisinstance, the magnetic field lines of the radially symmetric biasmagnetic field produced by the back bias magnet 5 are pulled in thez-direction towards the tooth 2. Thus, the magnetic field lines arepulled away from the x and y-axes (i.e., the sensor planes) and thesensed magnetic field strength in the x and y-directions is reduced suchthat a minimum field strength is detected at the center of the tooth 2.This may differ in real-world applications where the minimum may notoccur exactly at the center due to assembly tolerances, but the minimumfield strength should be detected substantially at the center of thetooth 2.

Conversely, FIG. 1B shows a notch 3 of wheel 1 passing the sensor module6. In this instance, the magnetic field lines of the radially symmetricbias magnetic field produced by the back bias magnet 5 are not pulled(or less pulled) in the z-direction towards the notch 3. Thus, themagnetic field lines remain concentrated relative to the x and y-axes(i.e., the sensor planes) and the sensed magnetic field strength in thex and y-directions are at a maximum at the center of the notch 3. Thismay differ in real-world applications where the maximum may not occurexactly at the center, but the maximum field strength should be detectedsubstantially at the center of the notch 3.

As the wheel 1 rotates, the teeth 2 and notches 3 alternate past thesensor module 6 and the sensor elements within the sensor arrangement 4sense a change in the x-axis and y-axis magnetic field strength thatvaries as a sinusoidal waveform (i.e., as a signal modulation), thefrequency of which corresponds to a speed of rotation of the wheel,which further corresponds to a speed of rotation of a drive shaft (e.g.,camshaft) that drives the rotation of the wheel. Thus, the sensorcircuit of the sensor arrangement 4 that receives signals (i.e., sensorsignals) from the magnetic field sensor elements and derives, from thesensor signals, a measurement signal that represents the magnetic fieldas a signal modulation. The measurement signal may then be output as anoutput signal to an external controller, control unit or processor(e.g., an ECU). The external device counts the pulses of the outputsignal and is able to calculate wheel-speed and an actual angle of therotating wheel.

FIG. 2 illustrates a plan view of the sensor arrangement 4 according toone or more embodiments. As used herein, the sensor arrangement 4 mayalso be referred to as a sensor chip layout, single die sensor ormagnetic sensor and includes at least four magnetic field sensorelements 10L, 10R, 10U, and 10D (collectively referred to as sensorelements 10), and a sensor circuit 8. The sensor elements 10 arearranged on a circumference of a circle 12 with equidistant spacing fromeach other. Thus, the sensor elements 10 are spatially distributedequally about a center axis 11 of the circle 12 such that all sensorelements 10 are exposed to substantially the same (due to typicalassembly tolerances of 3%), or exactly the same magnetic field workingpoint. For example, as noted above, the radially symmetric magneticfield produced by the axially polarized magnet 5 is zero at the centerof the magnet (i.e., at its center axis) and increases in a radialdirection from the center axis (e.g., from the z-axis as shown). Thus,the center point of the circle 12 is arranged to coincided with thecenter axis of the magnet 5 so that each sensor element 10 is exposed tosubstantially the same (due to typical assembly tolerances of 3%), orexactly the same magnetic field working point.

The sensor elements 10 may be, for example, single-axis or multi-axisXMR sensor elements that have a sensing axis utilized for the speedsensor that is aligned with one of the in-plane magnetic fieldcomponents Bx or By. Here, as similarly described above with referenceto FIGS. 1A and 1B, it is assumed for this example that the back biasmagnet 5 produces a radially symmetric bias magnetic field.Additionally, sensor's transfer function has a high linear range (+/−25mT) and is in a wide range independent from bias fields. That is, eachsensor element 10 is sensitive to a first magnetic in-plane fieldcomponent (e.g., a Bx component) and, at the same time, it isindependent from (or insensitive to) a second magnetic in-plan fieldcomponent (e.g., a By component).

The arrows on each sensor element 10 indicate a direction of thereference layer of the sensor element 10 having a reference directionsuch that the reference direction of sensor elements 10L, 10R are thesame and the reference direction of sensor elements 10U, 10D are thesame. Thus, sensor elements 10L and 10R share their same referencedirection, and sensor elements 10U and 10D share their own samereference direction. Moreover, the sign of the pairwise referencedirections is also invertible. This means in another embodiment, sensorelements 10L and 10R may also be sensitive to the −Bx direction, whilesensor elements 10U and 10D may be sensitive to the −By direction.Accordingly, if the magnetic field points exactly in the same directionas the reference direction, the resistance of the XMR sensor element isat a maximum, and, if the magnetic field points exactly in the oppositedirection as the reference direction, the resistance of the XMR sensorelement is at a minimum.

According to this example, oppositely disposed sensor elements 10L and10R may have a sensing axis in the x-direction configured for sensingthe in-plane magnetic field component Bx (i.e., sensitive to magneticfields in the x-plane). Similarly, oppositely disposed sensor elements10U and 10D may have a sensing axis in the y-direction configured forsensing the in-plane magnetic field component By (i.e., sensitive tomagnetic fields in the y-plane).

The sensor signals of each sensor element 10 is provided to the sensorcircuit 8 that calculates an output signal using a differentialcalculation that cancels out the homogeneous stray-fields in the x andy-directions, and out-of-plane magnetic field components do not affectthe output signal (i.e., the sensor output). The output signal R_(OUT)is calculated, for example, by the following equation:

R _(OUT) =R _(LEFT) −R _(RIGHT)−(R _(UP) −R _(DOWN))  (1), or

V _(OUT) =V _(LEFT) −V _(RIGHT)−(V _(UP) −V _(DOWN))  (2).

Here, R_(LEFT) corresponds to a resistance value of sensor element 10L,R_(RIGHT) corresponds to a resistance value of sensor element 10R,R_(UP) corresponds to a resistance value of sensor element 10U, andR_(DOWN) corresponds to a resistance value of sensor element 10D.Furthermore, V_(LEFT) corresponds to a voltage value of sensor element10L, V_(RIGHT) corresponds to a voltage value of sensor element 10R,V_(UP) corresponds to a voltage value of sensor element 10U, andV_(DOWN) corresponds to a voltage value of sensor element 10D. Equations(1) and (2) can be generalized as follows:

SE_(OUT)=SE_(A)−SE_(B)−(SE_(C)−SE_(D))  (3),

where SE corresponds to sensor element, and SE_(A) and SE_(B) correspondto a first pair of oppositely disposed sensor elements, and SE_(C) andSE_(D) correspond to a second pair of oppositely disposed sensorelements.

As the sensor elements 10 are XMR sensor elements, the resistance valueschange depending on the magnetic field strength in the direction of thesensing axis, and the resistance values of the XMR sensor elements maybe detected by the sensor circuit 8 or may be output from the sensorelement as a voltage value that is representative of the resistancevalue (i.e., the voltage value changes as the resistance value changes).In the former case, the resistance value is output as a sensor signal,and, in the latter case, the voltage value is output as a sensor signal,however, the sensor signal is not limited thereto. Thus, externalstray-fields in the sensor plane will cancel out due to the differentialcalculus and out-of-plane magnetic field components do not affect thesensor output.

Alternatively, the sensor elements 10 may be, for example, vertical Hallsensor elements (e.g., Hall plates) that have a sensing axis utilizedfor the speed sensor that is aligned with one of the in-plane magneticfield components Bx or By. In vertical Hall sensor elements, voltagevalues output by the sensor elements 10 change according to the magneticfield strength in the direction of the sensing axis. Thus, externalstray-fields in the sensor plane will cancel out due to the differentialcalculus and out-of-plane magnetic field components do not affect thesensor output.

Thus, the sensor elements 10 may be any sensor element sensitive to and,thus capable of detecting, a magnetic field in an in-plane direction.For example, oppositely disposed sensor elements 10L and 10R may have asensing axis aligned in the x-direction configured for sensing thein-plane magnetic field component Bx (i.e., sensitive to magnetic fieldsin the x-plane). Similarly, oppositely disposed sensor elements 10U and10D may have a sensing axis aligned in the y-direction configured forsensing the in-plane magnetic field component By (i.e., sensitive tomagnetic fields in the y-plane).

In addition, the sensor module 6 includes an axially polarized cylindermagnet, where its center axis points towards the wheel 1 and coincideswith center axis 11. Thus, the magnet creates a radially symmetric biasmagnetic field in the sensor plane such that each sensor element 10 isexposed to substantially the same (due to typical assembly tolerances of3%), or exactly the same magnetic field working point. The magnet may beany shape that produces a radially symmetric magnetic field (e.g.,cylinder, cube, etc.).

For example, FIG. 3 illustrates a back bias magnet 15 that is an axiallypolarized cylinder according to one or more embodiments. FIG. 3 furthershows the in-plane magnetic field distribution in the sensor plane. Themagnetic field is zero in the center of the plane and increases in theradial direction. Thus, due to the radially symmetric fielddistribution, all four sensor elements 10 are exposed to substantiallythe same (due to typical assembly tolerances of 3%), or exactly the samemagnetic field working point.

FIG. 4 illustrates a speed sensing system 400, including a sensor module16, according to one or more embodiments. In particular, a portion ofwheel 1 is shown with an air gap 17 between the wheel 1 and the sensormodule 16, and, more particularly, between the wheel 1 and the sensorarrangement 4. The sensor arrangement 4 is disposed on or coupled to thecylinder back bias magnet 15 such that the center point between thesensor elements 10 (e.g., the center 11 of circle 12) is aligned on thecenter point (i.e., the center axis) of the magnet 15. As describedabove, the sensor arrangement 4 (i.e., the sensor) includes magneticsensor elements 10 and an IC for signal conditioning.

FIGS. 5A-5D illustrate output signals of a sensor circuit of a sensorshown in FIG. 4 verse a rotation angle of a target wheel according toone or more embodiments. The x-axis, for example, illustrates a rotationangle of the target wheel, illustrating a partial revolution of thetarget wheel from 0° to 45°. FIGS. 5B and 5D are normalizedrepresentations of their counterpart graphs shown in FIGS. 5A and 5C,respectively.

In particular, the output signals illustrated in FIGS. 5A and 5B aredifferential sensor signals after applying one of equations (1) or (2)to the sensor signals of the sensor elements 10 with an air gap of 0.5mm. The shape of the target wheel (tooth 2 and notches 3) is representedby a rectangular shaped function in each graph. Different twist angles(0°, 45° and 90°) of the sensor module 16 around its z-axis aresuperimposed on each graph. The rotation of the target wheel starts, forexample, at 0° (at this step the sensor faces a notch 3), at 22.5° thesensor faces the middle of the tooth 2, and at 45° the sensor facesanother notch 3.

As can be seen, the twist of the sensor module 16 has little effect onthe output signals. In particular, the rotation of the target wheelmodulates the magnetic field, and a clear signal change (modulation) asa function of the wheel rotation angle is illustrated in the graphsshown in FIGS. 5A and 5B. However, the normalized curves for thedifferent twist angles nearly overlap each other in FIG. 5B. From thisobservation, it can be concluded that the twist of the sensor module 16around its z-axis has almost no effect on the output signal.

Furthermore, as can be observed in FIG. 5B, showing the normalizeddifferential signals of FIG. 5A, the output switching behavior switcheson the tooth edge where the output signal crosses the x-axis on thegraph. Alternatively, the sensor may be programmed to switch at anarbitrary threshold level of the output signal. For instance at 70% ofthe signal level. Thus, is it not stringent that the sensor switchesexactly the crossing with the x-axis.

Similarly, the output signals illustrated in FIGS. 5C and 5D aredifferential sensor signals after applying one of equations (1) or (2)to the sensor signals of the sensor elements 10 with an air gap of 2.5mm. Again, different twist angles (0°, 45° and 90°) of the sensor module16 around its z-axis are superimposed on each graph. The rotation of thetarget wheel starts, for example, at 0° (at this step the sensor faces anotch 3), at 22.5° the sensor faces the middle of the tooth 2, and at45° the sensor faces another notch 3.

Despite the increase in air gap compared to the air gap used in FIGS. 5Aand 5B, the twist of the sensor module 16 has little effect on theoutput signals. This phenomenon is observed from the nearly overlapping(normalized) curves for the different twist angles shown in FIG. 5D.

Furthermore, as can be observed in FIG. 5D, showing the normalizeddifferential signals of FIG. 5C, the output switching behavior switcheson the tooth edge where the output signal crosses the x-axis on thegraph. Alternatively, the sensor may be programmed to switch at anarbitrary threshold level of the output signal. For instance at 70% ofthe signal level. Thus, is it not stringent that the sensor switchesexactly the crossing with the x-axis.

In view of FIGS. 5A-5D, the output signal may be independent from themounting angle (i.e., independent of a twisting angle around itsz-axis). The sensor arrangement 4 may be robust against stray-fields dueto differential signal calculation that cancels out homogeneousstray-fields in both in-plane directions (i.e., the x and y-planes), andout-of-plane magnetic field components do not affect the output signal.The output signal of the sensor circuit 8 complies with output switchingon the tooth edge. Thus, there is no need to reconfigure an externalcontrol unit (e.g., an ECU) during installation. Furthermore, a simpleaxially polarized cylinder back bias magnet is sufficient. Accordingly,the described embodiments offer stray-field robust, twist-insensitivesensing of the wheel, and it comes with a low cost magnetic back biassolution (e.g., a sintered ferrite cylinder magnet). Alternatively,other types of magnets (e.g., a rare earth magnet) may also be suitableas a back bias magnet.

It is noted that, while the embodiments refer to four sensor elements10, any even number of N sensor elements of four or more may beimplemented, such that oppositely disposed sensor elements (i.e., eachoppositely disposed sensor element pair such as sensor elements 10R and10L or sensor elements 10U and 10D) have the same reference directionthat is parallel to a radial direction that intersects with the centeraxis 11 of the circle 12, and that the N sensor elements are aligned atequidistant angles about the circle 12.

FIG. 6 illustrates an alternative embodiment to the sensor system 600shown in FIG. 4. In particular, FIG. 6 shows a sensor system 600 thatincludes an magnetized encoder wheel 61 comprised of alternating northpole sections 62 and south pole sections 63. Accordingly, the north polesections 62 and south pole sections 63 represent teeth and notches of atooth and notch wheel described above. The sensor elements 10 of sensorarrangement 4, as described in reference to FIG. 2, are sensitive tomagnetic fields influenced by the north pole sections 62 and south polesections 63 of the wheel 61. Here, since the magnetic field is activelygenerated by the wheel 61, a back bias magnet can be omitted. Thus, thesensor circuit 8 of the sensor arrangement 4 generates a sensor outputthat corresponds to the rotational speed of the wheel 61 by detectingthe change of the alternating magnetic field.

The sensor circuit 8 may transmit the sensor output signal (i.e., thedifferential signal) to an external processor or controller unit, suchas an ECU for speed calculation and determination, which in turn mayprovide a speed measurement to a user or other processing or outputcomponent, such as a display. Alternatively, the sensor circuit 8 may bebypassed and the external processor or controller unit may receive thesensor signals from the sensor elements 10 for calculating thedifferential signal and calculating a wheel rotation speed from thedifferential signal.

FIG. 7 illustrates a sensor module 70 according to one or moreembodiments. In particular, FIG. 7 shows a differential lateral Hallsensor, and a magnetic back bias circuit that enables twist-insensitiveand stray-field robust sensing of the target wheel (e.g., a toothedwheel).

The sensor module 70 includes a sensor package 71 with lead frame 72extending therefrom, a back bias magnet 74 located on a back side of thesensor package 71 and magnetized in the (in-plane) x-direction, a firstmagnetic flux guide 76 a, and a second magnetic flux guide 76 b.

The sensor package 71 includes a first lateral Hall sensor element(e.g., Hall plate) 73 a, a second lateral Hall sensor element (e.g.,Hall plate) 73 b, and a sensor circuit (not shown). The first lateralHall sensor element 73 a and the second lateral Hall sensor element 73 b(commonly referred to as sensor elements 73) have a sensitivity-axisaligned parallel to the z-axis, which is an out-of-plane component ofthe sensor package 71 and are sensitive to magnetic field component Bz(i.e., the magnetic field in the z-plane). Here, a back side of thesensor package 71 refers to the side that is furthest from the targetwheel and a front side of the sensor package 71 faces the target wheelin the z-direction.

The back bias magnet 74 is magnetized in the x-direction, parallel tothe in-plane component of the sensor package 71. The back bias magnet 74may be, for example, a block or cylinder magnet placed between the afirst magnetic flux guide 76 a, and a second magnetic flux guide 76 b,and coupled to the back side of the sensor package 71.

The first magnetic flux guide 76 a and the second magnetic flux guide 76b are located at opposite poles of the magnet 74 and made of a material(e.g., iron) capable of redirecting the magnetic field produced by themagnet 74. In particular, FIG. 8 illustrates a plan view of the sensormodule 70 showing magnetic field patterns coupled in a loop 77 in theenvironment around the sensor module 70. As the magnet 74 is magnetizedin the x-direction, the magnetic B-field (flux lines) starts in anx-direction at point 77 a, a portion of the magnetic B-field isredirected by the second magnetic flux guide 76 b at points 77 b and 77c such that the magnet B-field is directed, anti-parallel to thex-direction, through the (in-plane) x-plane of the sensor package 71 atpoint 77 d, and a portion of the magnetic B-field is redirected again bythe first magnetic flux guide 76 a at points 77 e and 77 f such that themagnet B-field is directed back in the x-direction.

In absence of the ferrous target wheel (or in front of a notch), themagnetic B-field will exit from the second magnetic flux guide 76 b anddirectly couple into the first magnetic flux guide 76 a again. Thus,there is a strong negative Bx-field and almost no Bz or By field at thesensor location (i.e., at the sensor elements 73). Thus, theBz-sensitive Hall plates are exposed to low Bz-fields, with a smalloffset. The Bz-field on the first lateral Hall sensor element 73 a andsecond lateral Hall sensor element 73 b are measured with opposite sign.For example, the sensor signal generated by the first lateral Hallsensor element 73 a may be a low value (e.g., corresponding to −8 mT),and the sensor signal generated by the second lateral Hall sensorelement 73 b may be a low value with opposite sign (e.g., correspondingto +8 mT). Thus, the absolute values of the sensor signals are equal.

Conversely, in the presence of the wheel (or in front of a tooth), themagnetic field will be pulled in z-direction towards the target wheel.The Bz-field on the first lateral Hall sensor element 73 a and secondlateral Hall sensor element 73 b increases with opposite sign due to theincrease in Bz-field strength. That is, as the sensor signal generatedby the first lateral Hall sensor element 73 a becomes more negative(e.g., corresponding to −11 mT), the sensor signal generated by thesecond lateral Hall sensor element 73 b becomes more positive by thesame amount (e.g., corresponding to +11 mT). Thus, the absolute valuesof the sensor signals are equal.

In this way a signal modulation can be obtained by the sensor circuit asthe wheel rotates in front of the sensor module 70. The modulation isalways present, independent from the mounting angle (i.e., e.g., thesensor rotation around its z-axis), and robust against stray-fields asthey are canceled out by the differential configuration of the sensorelements 73. It should be noted that it may be preferable that themid-point between sensor elements 73 should be placed on a lateralmid-point of the magnet 74 such that the (absolute) opposite sign valuesof the sensor signals are maintained equal.

FIGS. 9A-9D illustrate output signals of a sensor circuit of a sensorshown in FIG. 7 verse a rotation angle of a target wheel according toone or more embodiments. The x-axis, for example, illustrates a rotationangle of the target wheel, illustrating a partial revolution of thetarget wheel from 0° to 45°. FIGS. 9B and 9D are normalizedrepresentations of their counterpart graphs shown in FIGS. 9A and 9C,respectively.

In particular, the output signals illustrated in FIGS. 9A and 9B aredifferential sensor signals after applying a differential equation(e.g., BzLEFT (73 a)-BzRIGHT (73 b)) to the sensor signals of the sensorelements with an air gap of 0.5 mm. The shape of the target wheel (tooth2 and notches 3) is represented by a rectangular shaped function in eachgraph. Different twist angles (0° and 90°) of the sensor module 70around its z-axis are superimposed on each graph. The rotation of thetarget wheel starts, for example, at 0° (at this step the sensor faces anotch 3), at 22.5° the sensor faces the middle of the tooth 2, and at45° the sensor faces another notch 3.

As can be seen, the twist of the sensor module 70 has little effect onthe output signals. In particular, the rotation of the target wheelmodulates the magnetic field, and a clear signal change (modulation) asa function of the wheel rotation angle is illustrated in the graphsshown in FIGS. 9A and 9B. However, the normalized curves for thedifferent twist angles nearly overlap each other in FIG. 9B. From thisobservation, it can be concluded that the twist of the sensor module 70around its z-axis has almost no effect on the output signal.

Furthermore, as can be observed in FIG. 9B, showing the normalizeddifferential signals of FIG. 9A, the output switching behavior switcheson the tooth edge where the output signal crosses the x-axis on thegraph. Alternatively, the sensor may be programmed to switch at anarbitrary threshold level of the output signal. For instance at 70% ofthe signal level. Thus, is it not stringent that the sensor switchesexactly the crossing with the x-axis.

Similarly, the output signals illustrated in FIGS. 9C and 9D aredifferential sensor signals after applying a differential equation(e.g., BzLEFT (73 a)-BzRIGHT (73 b)) to the sensor signals of the sensorelements with an air gap of 2.5 mm. Again, different twist angles (0°and 90°) of the sensor module 70 around its z-axis are superimposed oneach graph. The rotation of the target wheel starts, for example, at 0°(at this step the sensor faces a notch 3), at 22.5° the sensor faces themiddle of the tooth 2, and at 45° the sensor faces another notch 3.

Despite the increase in air gap compared to the air gap used in FIGS. 9Aand 9B, the twist of the sensor module 70 has little effect on theoutput signals. This phenomenon is observed from the nearly overlapping(normalized) curves for the different twist angles shown in FIG. 9D.

Furthermore, as can be observed in FIG. 9D, showing the normalizeddifferential signals of FIG. 9C, the output switching behavior switcheson the tooth edge where the output signal crosses the x-axis on thegraph. Alternatively, the sensor may be programmed to switch at anarbitrary threshold level of the output signal. For instance at 70% ofthe signal level. Thus, is it not stringent that the sensor switchesexactly the crossing with the x-axis.

The sensor circuit may transmit the sensor output signal (i.e., thedifferential signal) to an external processor or controller unit, suchas an ECU for speed calculation and determination, which in turn mayprovide a speed measurement to a user or other component. Alternatively,the sensor circuit may be bypassed and the external processor orcontroller unit may receive the sensor signals from the sensor elements73 for calculating the differential signal and calculating a wheelrotation speed from the differential signal.

FIG. 10 illustrates a flow diagram of a method 1000 of measuring arotational speed of a rotating member by a magnetic sensor according toone or more embodiments. As noted above, the magnetic sensor including aplurality of sensor elements arranged in a sensor plane of the magneticsensor and are exposed to a substantially same working point of aradially symmetric bias magnetic field produced by an axially polarizedback bias magnet. The method includes generating measurement values by aplurality of sensor elements in response to sensing the radiallysymmetric bias magnetic field (operation 5). The variations in themeasurement values of the plurality of sensor elements are caused by arotation of the rotating member. The method further includes generatinga measurement signal using a differential calculation with themeasurement values as inputs to the differential calculation (operation10). The differential calculation, performed by a processor, isconfigured to, based on the measurement values, cancel out stray-fieldsin both a direction of a first sensitivity-axis of a first pair ofsensor elements and a direction of a second sensitivity-axis of a secondpair of sensor elements. The first pair of sensor elements are sensitiveto a first in-plane magnetic field component of the radially symmetricbias magnetic field in the direction of the first sensitivity-axis, andthe second pair of sensor elements are sensitive to a second in-planemagnetic field component of the radially symmetric bias magnetic fieldin the direction of the second sensitivity-axis. Accordingly, themeasurement signal oscillates between maximum and minimum values basedon a rotational speed of the rotating member. Lastly, the methodincludes outputting the measurement signal to an external device(operation 15), such as an ECU, for further processing. The measurementsignal may be output by transmission along a wired connection or awireless connection.

In view of the above, assembly tolerances can be relaxed by implementingthe sensor modules described above due to their twist-insensitivity andstray-field robustness.

While the above embodiments are described in the context of detecting awheel or camshaft speed, the sensor may be used to detect the rotationspeed of any rotating member or object that creates sinusoidalvariations in a magnetic field as it rotates and that may be sensed by asensor. For example, a combination of a ferrous wheel and a back biasmagnet may be used to generate a time varying magnetic field.Alternatively, an active encoder wheel (without a back bias magnetic)may be used to generate a time varying magnetic field.

Further, while various embodiments have been described, it will beapparent to those of ordinary skill in the art that many moreembodiments and implementations are possible within the scope of theinvention. Accordingly, the invention is not to be restricted except inlight of the attached claims and their equivalents. With regard to thevarious functions performed by the components or structures describedabove (assemblies, devices, circuits, systems, etc.), the terms(including a reference to a “means”) used to describe such componentsare intended to correspond, unless otherwise indicated, to any componentor structure that performs the specified function of the describedcomponent (i.e., that is functionally equivalent), even if notstructurally equivalent to the disclosed structure that performs thefunction in the exemplary implementations of the invention illustratedherein.

Furthermore, the following claims are hereby incorporated into thedetailed description, where each claim may stand on its own as aseparate example embodiment. While each claim may stand on its own as aseparate example embodiment, it is to be noted that—although a dependentclaim may refer in the claims to a specific combination with one or moreother claims—other example embodiments may also include a combination ofthe dependent claim with the subject matter of each other dependent orindependent claim. Such combinations are proposed herein unless it isstated that a specific combination is not intended. Furthermore, it isintended to include also features of a claim to any other independentclaim even if this claim is not directly made dependent to theindependent claim.

It is further to be noted that methods disclosed in the specification orin the claims may be implemented by a device having means for performingeach of the respective acts of these methods.

Further, it is to be understood that the disclosure of multiple acts orfunctions disclosed in the specification or claims may not be construedas to be within the specific order. Therefore, the disclosure ofmultiple acts or functions will not limit these to a particular orderunless such acts or functions are not interchangeable for technicalreasons. Furthermore, in some embodiments a single act may include ormay be broken into multiple sub acts. Such sub acts may be included andpart of the disclosure of this single act unless explicitly excluded.

What is claimed is:
 1. A magnetic sensor module comprising: a magneticsensor having an in-plane axis and an out-of-plane axis, the magneticsensor including a differential pair of sensor elements spaced apartfrom each other, wherein the differential pair of sensor elements areconfigured to generate measurement values in response to sensing a biasmagnetic field; a back bias magnet comprising two opposing poles,including a first pole and a second pole, wherein the back bias magnetis magnetized in a magnetized direction that is parallel to the in-planeaxis and generates the bias magnetic field; a first magnetic flux guidedisposed at the first pole and configured to redirect a first portion ofthe bias magnetic field towards the magnetic sensor along the in-planeaxis; and a second magnetic flux guide disposed at the second pole andconfigured to redirect a second portion of the bias magnetic fieldtowards the back bias magnet along the magnetized direction.
 2. Themagnetic sensor module of claim 1, wherein flux lines of the biasmagnetic field from the redirected first portion of the bias magneticfield and flux lines of the bias magnetic field from the redirectedsecond portion of the bias magnetic field are coupled in a loop.
 3. Themagnetic sensor module of claim 1, wherein the first magnetic flux guideis configured to redirect the first portion of the bias magnetic fieldto pass through the differential pair of sensor elements.
 4. Themagnetic sensor module of claim 1, wherein the differential pair ofsensor elements are spaced apart from each other along the in-plane axisand are configured to sense the bias magnetic field along theout-of-plane axis.
 5. The magnetic sensor module of claim 4, wherein thedifferential pair of sensor elements are lateral Hall sensor elements.6. The magnetic sensor module of claim 4, wherein the measurement valuesgenerated by the differential pair of sensor elements are substantiallyequal in magnitude with opposing signs.
 7. The magnetic sensor module ofclaim 4, wherein a mid-point between the differential pair of sensorelements is aligned with a midpoint of the back bias magnet.
 8. Themagnetic sensor module of claim 1, wherein the differential pair ofsensor elements are disposed at a first side of the magnetic sensor andthe back bias magnet is coupled to the magnetic sensor at a second sideof the magnetic sensor that is opposite to the first side of themagnetic sensor.
 9. The magnetic sensor module of claim 1, wherein thedifferential pair of sensor elements are configured to measure anout-of-plane component of the bias magnetic field, and the magneticsensor is configured to detect a rotation of an object such that theout-of-plane component of the bias magnetic field oscillates between twoextrema as the object rotates.
 10. The magnetic sensor module of claim1, wherein the magnetic sensor is configured to generate a differentialmeasurement signal based on the measurement values generated by thedifferential pair of sensor elements.
 11. The magnetic sensor module ofclaim 10, wherein the magnetic sensor is configured to generate thedifferential measurement signal such that an external magnetic field iscanceled out.
 12. The magnetic sensor module of claim 10, wherein themagnetic sensor is configured to generate the differential measurementsignal independent of a mounting angle of the magnetic sensor moduleabout the out-of-plane axis relative to the object.
 13. The magneticsensor module of claim 10, wherein the magnetic sensor is configured todetect a rotation of an object such that the differential measurementsignal oscillates between two extrema as the object rotates.
 14. Themagnetic sensor module of claim 13, wherein: the magnetic sensor moduleis configured such that a field strength of the out-of-plane componentof the bias magnetic field changes according to the rotation of theobject that has alternating first portions and second portions, and theout-of-plane component of the bias magnetic field is stronger whenproximate to one of the first portions and weaker when proximate to oneof the second portions.
 15. The magnetic sensor module of claim 13,wherein the object is a toothed wheel, and the first portions are teethand the second portions are notches of the toothed wheel.
 16. Themagnetic sensor module of claim 13, wherein a frequency of thedifferential measurement signal is proportional to a rotational speed ofthe object.
 17. A method of measuring a rotational speed of an object bya magnetic sensor, the method comprising: generating a bias magneticfield that is directed in parallel to an in-plane axis of the magneticsensor; redirecting a first portion of the bias magnetic field towardsthe magnetic sensor along the in-plane axis; redirecting a secondportion of the bias magnetic field such that flux lines of the biasmagnetic field from the redirected first portion of the bias magneticfield and flux lines of the bias magnetic field from the redirectedsecond portion of the bias magnetic field are coupled in a loop;generating measurement values by a differential pair of sensor elementsin response to sensing an out-of-plane component of the bias magneticfield, wherein variations in the measurement values generated by thedifferential pair of sensor elements are caused by a rotation of theobject; generating a differential measurement signal based on themeasurement values generated by the differential pair of sensorelements, wherein the differential measurement signal oscillates betweentwo extrema as the object rotates; and outputting the differentialmeasurement signal.
 18. The method of claim 17, wherein redirecting thefirst portion of the bias magnetic field includes redirecting the firstportion of the bias magnetic field to pass through the differential pairof sensor elements.
 19. The method of claim 17, wherein a field strengthof the out-of-plane component of the bias magnetic field changesaccording to the rotation of the object that has alternating firstportions and second portions, and the out-of-plane component of the biasmagnetic field is stronger when proximate to one of the first portionsand weaker when proximate to one of the second portions.
 20. The methodof claim 17, wherein a frequency of the differential measurement signalis proportional to the rotational speed of the object.